Precision optical elements are a necessity in many different fields including high powered laser research, astronomy, and numerous other commercial and military areas. Unfortunately, many of the current optical element fabrication techniques, in which the optical element is held or secured to a blocking plate during the fabrication of the optical element, often result in deformation of the optical element. One of the current techniques used to hold the optical element during fabrication is referred to as "Cold-Blocking."
An example of one type of cold-blocking is set forth in the "Optical Activities in Industry" article by Frank Cooke published in Applied Optics, Vol. 2, No. 9, pages 978-979, September 1963. In the article, a glass base plate is covered by a layer of a double-sided tape. Mirrors are then mounted on the base plate with the double-sided tape holding the mirrors in place during grinding, lapping, and polishing. After the machining of the mirrors is completed, the mirrors are removed from the glass base plate and the double-sided tape. However, as stated in the Cooke article, this technique only functions well for "moderately accurate parts."
More recently, cold-blocking techniques have been developed for use with very. accurate and high aspect ratio parts. High aspect ratio elements are commonly defined as those optical elements having a thickness to diameter ratio of greater than about 1:5. These techniques typically involve the use of one of two kinds of cold adhesive. The first of the two types is a thermosetting adhesive. The thermosetting adhesive is applied to the surface of a blocking tool similar to the base plate of the Cooke article. The optical element is placed on the blocking tool with the layer of thermosetting adhesive located between the tool and the element. After the thermosetting cures, the blocked optical element is machined, that is, grinded, lapped, and polished. The blocked element is then subjected to a debonding solvent, and the blocked element is removed from the tool. Unfortunately, due to the shrinkage coefficient of the thermosetting adhesive, the adhesive tends to cause deformation of the optical element as the adhesive cures thereby rendering the optical element unsuitable for precision applications.
The second type of adhesive frequently used in cold-blocking is a UV cured adhesive. The use of such UV adhesives was considered and discussed during an Optical Fabrication & Testing Workshop sponsored by the Optical Society of America held in Boston, Mass. from Nov. 17-19, 1992. A paper entitled "Lens Blocking Method for Opticam" by Robert Novak et al. published in the 1992 Technical Digest Series, Vol. 24, at pages 245-251, discussed a specific type of cold blocking employing a UV adhesive, and a specially designed blocking tool. However, the blocking tool described in the Novak et al. article does not allow for simultaneous blocking of multiple optical elements. Furthermore, the device set forth in the Novak et al. article does not provide support to the entire surface of the blocked element. Instead, the device of the Novak et al. article only provides peripheral or edge located support to optical elements placed thereon.
A second paper dealing with cold-blocking and the use of UV adhesives entitled "Ultraviolet Light Immobilized Lens Blocking Adhesive Performance Quantified Using Polymer Analysis Techniques" by Fred Caputo et al. was also published in the 1992 Technical Digest Series, Vol. 24, at pages 252-263.
As set forth in the above-mentioned articles, typically, a layer of the UV adhesive is applied to the top surface of the blocking tool, and the optical element to be blocked is placed on the blocking tool. The block, that is, the blocking tool along with the blocked optical element, is then exposed to ultra-violet light. The UV adhesive cures when exposed to the ultra-violet light thereby securing the optical element to the blocking tool for subsequent machining of the blocked element. The blocked element is then subjected to a debonding solvent, and the blocked element is removed from the tool. Unfortunately, due to nature of the UV adhesive, and the fact that the solvent must penetrate between the blocking tool and the optical element and completely through the layer of the UV adhesive the block must soak for in the solvent from several days to several weeks. Thus, the extended deblocking time renders the use of UV adhesives cumbersome.
Additionally as the layer of optical adhesive cures, a strain is produced on the blocked optical element. The strain can result in deformation of the blocked element. Thus, high aspect ratio optical elements are particularly susceptible to deformation caused by the strain generated during curing of the UV adhesive.
All types of optical blocking have several requirements in common. The optical adhesive must be easy to apply and must not cause deformation of the blocked optical elements. Furthermore, it is desired that the blocked element may be easily removed or "deblocked" after subsequent machining of the blocked optical element.
Consequently, as the technological demands for very precise optical elements increase, a need exists for a technique to hold optical elements during their fabrication without causing deformation, which does not require excessive deblocking times, and which can be used with very accurate and high aspect ratio optical elements.